[Bug libstdc++/108822] [C++23] Implement P2255R2, type trait to detect reference binding to temporary

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108822

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:f8a5298c97c460d45e888b123fe1bbcdb49b8ad4

commit r14-7225-gf8a5298c97c460d45e888b123fe1bbcdb49b8ad4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jan 10 17:29:22 2024 +0000

    libstdc++: Implement P2255R2 dangling checks for std::tuple [PR108822]

    This is the last part of PR libstdc++/108822 implementing P2255R2, which
    makes it ill-formed to create a std::tuple that would bind a reference
    to a temporary.

    The dangling checks are implemented as deleted constructors for C++20
    and higher, and as Debug Mode static assertions in the constructor body
    for older standards. This is similar to the r13-6084-g916ce577ad109b
    changes for std::pair.

    As part of this change, I've reimplemented most of std::tuple for C++20,
    making use of concepts to replace the enable_if constraints, and using
    conditional explicit to avoid duplicating most constructors. We could
    use conditional explicit for the C++11 implementation too (with pragmas
    to disables the -Wc++17-extensions warnings), but that should be done as
    a stage 1 change for GCC 15 rather than now.

    The partial specialization for std::tuple<T1, T2> is no longer used for
    C++20 (or more precisely, for a C++20 compiler that supports concepts
    and conditional explicit). The additional constructors and assignment
    operators that take std::pair arguments have been added to the C++20
    implementation of the primary template, with sizeof...(_Elements)==2
    constraints. This avoids reimplementing all the other constructors in
    the std::tuple<T1, T2> partial specialization to use concepts. This way
    we avoid four implementations of every constructor and only have three!
    (The primary template has an implementation of each constructor for
    C++11 and another for C++20, and the tuple<T1,T2> specialization has an
    implementation of each for C++11, so that's three for each constructor.)

    In order to make the constraints more efficient on the C++20 version of
    the default constructor I've also added a variable template for the
    __is_implicitly_default_constructible trait, implemented using concepts.

    libstdc++-v3/ChangeLog:

            PR libstdc++/108822
            * include/std/tuple (tuple): Add checks for dangling references.
            Reimplement constraints and constant expressions using C++20
            features.
            * include/std/type_traits [C++20]
            (__is_implicitly_default_constructible_v): Define.
            (__is_implicitly_default_constructible): Use variable template.
            * testsuite/20_util/tuple/dangling_ref.cc: New test.

    Reviewed-by: Patrick Palka <ppalka@redhat.com>